use precise memory for handle callbacks for c++

[Alexander Gallego]

codegen:c++

...
private:
  std::vector<smf::rpc_service_method_handle> handles_;

should be followed by a handles_.reserve( $HandlesSize$ ); in the codegen

will will guarantee exact memory use.

[Alexander Gallego]

in fact it shhould be a std::array<> since we know the size.